@user unknown: Rather than leaving another comment, you could have just removed the sentence that obviously got there during the last edit accidentally, even though it was meant for somewhere else. Same goes for the user that upvoted the comment. Or you could have politely said that there is a sentence that does not seem to make sense at least.
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OregonGhostNov 14 '11 at 9:18

@OregonGhost: I didn't look into the edit history, so it wasn't obvious to me. But when you edit a question, you should remove greetings and similar boilerplate too.
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user unknownNov 14 '11 at 9:24

1

@userunknown: It was my fault, I pasted a sentence from elsewhere into the question unintentionally. I recommend to talk a little bit more kindly to new users.
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user508Nov 14 '11 at 19:59

If the noun to which the preposition is related has already been introduced or needs for other reasons not to be emphasized (e.g., if it is not important which one it is), it is better to use a contraction with a less "emphasizing" character:

Ich bin zum Bäcker gegangen.

but

An dem Tag habe ich schon etwas vor.

If you are in doubt, I would advise not using contractions, as they tend to sound a little "uncouth" if overused.

In many languages, prepositions and articles are sometimes contracted.

Obligatory contractions:

an + dem + noun → am + noun

in + dem + noun → im + noun

zu + dem + noun → zum + noun

zu + der + noun → zur + noun

Contractions that may occur in informal speech:

vor + dem + noun → vorm + noun

As for choosing between different cases:
If the case comes after a verb, you have to know what case the verb demands.

This corresponds to the knowledge of deciding if the correct English sentence is "I touch you." or "I touch to you." and very often, the construction with "to" corresponds to dative in German and without it to accusative in German.
In the light of the next phrase it can be more helpful to think of the action being done "for" someone when the dative is used.

For prepositions again, you should learn what case they demand, but an important rule is:
If the meaning is where something is, then the object has to take the dative, if the meaning is where it moves to, then the object has to take the accusative.
Note that this deviates from the correspondence with "to" in the previous paragraph.

It would be good to ask a more specific question if you want to know more.

Schiller has also used "ueberm (Sternenzelt)" as far as I recall. Is it normal in colloquial speech?
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Armen TsirunyanJan 30 '12 at 15:59

@ArmenTsirunyan Where I live not only "überm" but also "übern" is used. Also "unterm"/"untern", "durchn", "in'n", "mitm", "miter" (for "mit der") and so on are used.
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Alexander KosubekAug 8 '13 at 9:42

So for feminine nouns, "zur" works, but if you preposition was "in" then your are SOoL and have to use "in der"? Or is that just wrong?
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Adrian RatnapalaSep 12 '14 at 23:03